Catabolism of the essential amino acid tryptophan by the inducible heme-containing enzyme indoleamine 2,3-dioxygenase 1 (IDO1) is a central pathway maintaining the immunosuppressive microenvironment in many cancers. IDO1 catalyzes the degradation of tryptophan to kynurenine, and its effects on immune suppression are due to decreased tryptophan availability and the generation of tryptophan metabolites resulting in multipronged negative effects on cytotoxic T lymphocytes, as well as expansion of immunosuppressive regulatory T cells. IDO1 is elevated in multiple cancers, is induced by chemotherapy, targeted therapy, or immunotherapy. IDO1 expression in the tumor microenvironment is correlated with poor prognosis in a variety of cancers. IDO1 inhibitors are positioned to potentiate the efficacy of multiple oncology therapeutics including immunotherapies, targeted agents, and chemotherapies. Indeed, epacadostat (INCB24360), a potent and selective IDO1 inhibitor, entered clinical trials and is demonstrating activity in combination with ipilimumab (anti-CTLA4) in melanoma.
In addition to the above, IDO1 has been shown to play a role in chronic infections, HIV and AIDS, autoimmune diseases or disorders (e.g., rheumatoid arthritis), and immunologic tolerance, prevention of fetal rejection in utero. Inhibition of IDO1 may also be an important treatment strategy for patients with neurological or neuropsychiatric diseases or disorders, such as depression.
A need remains for additional therapeutic agents useful to treat proliferative disorders or diseases that are mediated by IDO1.